ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 271 then, either in the refrangihility or in the state of polarization we are to look for an explanation of the phenomenon. ! 17' 5. Regarding it at first as an axiom that the dispersed light |? of any given refrangibility could only have arisen from light of the same refrangibility contained in the incident beam, I was led :f|; to look in the direction of polarization for the required change <L* in the nature of the light. Since a fluid has no axes, circular polarization is the only kind which can here come into play. As some fluids are doubly refracting, transmitting right-handed and left-handed circularly polarized light with different velocities, so, it might be, this fluid was doubly absorbing, absorbing say right-handed circularly polarized light of certain refrangibilities with great energy, and freely transmitting left-handed. The right-handed light, absorbed, in the sense of withdrawn from the incident beam, might have been more strictly speaking scattered, and thereby depolarized. The common light so produced would be equivalent to two streams, of equal intensity, one of right-handed, and the other of left-handed circularly polarized light. Of these the latter would be freely transmitted, while the former would be scattered anew, and so on. Yet this hypothesis, sufficiently improbable already, was not enough. New suppositions were still required, to account for the circumstance that an epi-polized beam, when subjected to prismatic analysis with a low magnifying power, exhibited no bands of absorption in the region to which, as regards their refrangibility, the dispersed rays prin-cipally belong; so that altogether this theory bore not the slightest semblance of truth. 6. I found myself thus fairly driven to suppose that the change of nature consisted in a change of refrangibility. From the time of Newton it had been believed that light retains its refrangibility through all the modifications which it may undergo. in tint, just as musical notes of given pitch differ in quality. Were it not for the strong conviction I felt that liglit of definite refrangibility is in the strict sense of the word homogeneous, I should probably have been led to look in this direction for an explanation of the remarkable phenomena presented by a solution of sulphate of quinine. It would lead me too far from the subject of the present paper to explain the grounds of this conviction. I will only observe that I have not overlooked the remarkable effect of absorbing media in causing apparent changes of colour in a pure spectrum; but this I believe to be a subjective phenomenon, depending upon contrast.